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Capillary Exchange
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Capillary Exchange

Capillary Function
Molecular Exchange in Capillaries
Diffusion: movement of solutes such as oxygen and carbon dioxide. Filtration: movement of fluids and solutes from blood to interstitial space. Reabsorption: movement of fluids and solutes from interstitial space to blood. Diffusion is discreet movement of individual solutes; Filtration and Reabsorption are mass movements of fluids.
Interstitial Space:
Intermediate space between vascular system and tissues; comprises fluid that bathes tissue cells and vascular walls.
Diffusion
Review Diffusion.
Oxygen Gradient: Arterial -> Venous end of capillary
Blood: High oxygen (arterial) -> Low oxygen (venous)
Interstitial space: Low oxygen (arterial) -> High oxygen (venous)
Arterial system delivers oxygen rich blood to oxygen poor tissues; Venous system returns oxygen poor blood to heart/lungs.
Carbon Dioxide Gradient: Arterial -> Venous end of capillary
Blood: Low carbon dioxide (arterial) -> High carbon dioxide (venous)
Interstitial space: High carbon dioxide (arterial) -> Low carbon dioxide (venous)
Tissues release carbon dioxide as metabolic byproduct of cellular respiration; Venous system returns carbon dioxide to the lungs where it is exhaled.
Filtration & Reabsorption
Filtration:
Force provided by blood pressure (hydrostatic pressure). Fluid is pushed out of the capillary and into interstitial space. Occurs on arterial end: blood pressure > osmotic pressure.
Reabsorption:
Force provided by osmotic pressure. Plasma proteins remain in blood and create concentration gradient; Facilitate reabsorption of fluids into circulatory system. Occurs on venous end: osmotic pressure > blood pressure.
Lymphatic System:
Absorbs fluid lost by capillaries (~4L per day). Returns fluid to circulation, drains into venae cava.
Edema:
Imbalance in hydrostatic and osmotic forces results in too much fluid in interstitial space. Occurs in congestive heart failure, renal failure and liver failure. Results in tissue swelling.

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